In NC-EDM, a desired extent of machining, for example, in the direction of depth of a workpiece is preprogrammed and numerical values corresponding to successive relative positions between the tool electrode and the workpiece are stored in an NC unit and used to produce command signals for executing the desired relative displacement. Thus, under the commands of the NC unit the tool electrode and the workpiece are relatively displaced while a succession of effective electrical discharges are developed between the tool electrode and the workpiece to advance EDM stock removal. When the commanded machining displacement coincides with the recorded final position, the displacement may be terminated.
In "wear" mode which is often preferred to "no wear" mode in the interest of achieving greater stock removal or erosion, the tool electrode erodes as well. Thus, with the electrode erosion or wear positively exploited, the programmed extent of relative displacement becomes deficient and need to be compensated for the amount of electrode wear in the direction of displacement. Thus, the operator is then required to measure the actual depth of stock removal and subsequently to undertake the corrective machining step. This procedure is naturally inefficient and inaccurate. While a variety of electrode wear compensation techniques have been proposed, it has been found that none of them is truly satisfactory.